1. Technical Field
The present invention relates to fluid-dynamic-pressure bearings employed in signal record-and-playback devices in hard-disk drives and like apparatus, and to methods and equipment for manufacturing such bearings.
2. Description of the Related Art
Various fluid-dynamic-pressure bearings have been used to date in spindle motors employed in signal record-and-playback devices in hard-disk drives and like apparatus. “Fluid-dynamic-pressure bearings” are bearings in which a lubricating fluid such as oil is interposed between a shaft and a sleeve, wherein fluid pressure produced in the lubricating fluid is the bearing force.
One example of a spindle motor employing conventional pressure bearings of this sort is illustrated in FIG. 1. In between the outer circumferential surface of a shaft 2 that forms a unit with a rotor 1, and the inner circumferential surface of a sleeve 3 through which the shaft 2 is passed free to rotate, this spindle motor is configured with a pair of radial bearing sections 4, 4 that are separated in the axial direction. In between the upper surface of a discoid thrust plate 5 that projects radially outward from the circumferential surface of the shaft 1 on one of its ends, and the flat surface of a step recessed into the sleeve 3, as well as in between the undersurface of the thrust plate 5 and a thrust bush 6 that closes off one of the openings in the sleeve 3, the motor is also configured with a pair of respective thrust bearing sections 7, 7.
A single, continuous micro-gap is formed in between the shaft 2 and thrust plate 5, and the sleeve 3 and thrust bush 6. Oil 9 as a lubricating fluid is retained continuously without interruption through the course of the micro-gap. (This sort of oil-retaining structure will be denoted a “full-filled structure” hereinafter.)
Herringbone grooves 41, 41 and 71, 71 are formed in the radial bearing sections 4, 4 and the thrust bearing sections 7, 7. The herringbone grooves have a form in which pairs of spiral striations that act in mutually opposing directions are linked. In response to the rotor 1 rotating, maximum dynamic pressure is generated in the spiral-striation joints; the rotor 1 is supported by this dynamic pressure.
In a spindle motor of this sort, a taper-seal area 8 is formed alongside a portion of the sleeve 3 at its upper end, located on the motor end axially opposite the thrust bearing sections 7, 7. The oil-air interface is located in the taper-seal area 8. Because the oil has affinity for the wall surfaces of the taper-seal area, the interface is arcuate in cross-section.
Methods and apparatuses that have been proposed for charging bearing sections with the oil 9, retained as in the foregoing bearing configuration in between the thrust plate 5 and shaft 2, and the sleeve 3 and thrust bush 6, may be divided grossly into the following two methods and apparatuses.
The first method is one in which under an atmospheric-pressure environment a suitable amount of oil is put in the bearing opening, where the taper-seal area 8 or the like of the bearing unit is; the pressure is thereafter reduced to replace the air present in the bearing gap with oil; air bubbles in the oil are sufficiently discharged by leaving the bearing unit persisting a predetermined time under the reduced-pressure environment; and subsequently the environment surrounding the bearing is returned to atmospheric pressure. Japanese Pat. App. Pub. Nos. 2002-005170 and 2002-174242 are examples of prior art based on this method.
An example of the second method is: putting an oil pool and the bearings under a reduced-pressure environment from the start; under the reduced-pressure environment, putting a suitable amount of oil in the bearing opening, where the taper-seal area 8 or the like of the bearing unit is; exploiting capillary action to introduce the oil into the bearing interior; and thereafter returning the environment surrounding the bearing to atmospheric pressure. U.S. Pat. No. 5,524,729 and Japanese Pat. App. Pub. No. 2002-174243 are examples of prior literature pertaining to this method.
Nevertheless, there are problems such as the following with oil-charging methods as described above.
With the foregoing first oil-charging method, because the oil put in the taper-seal area 8 covers over the bearing opening, during the subsequent pressure reduction the air that had filled the bearing gap turns into air bubbles and passes into the oil. Under the circumstances, some of the oil splashes and splatters around the bearing, therefore making a process step of wiping up the splashes imperative. Because the wiping-up step is directly linked to rise in manufacturing cost, this method is not preferred. In some cases, moreover, if there is a threaded hole or the like in the vicinity of the bearing opening, the spattered oil invades the threads, making wipe-up impossible. Cases where the fluid-dynamic-pressure bearings have the structure given herein mean that this first oil-charging method cannot be used.
What is more, in the prior literature involving the first method, nothing is set forth regarding a way to adjust the amount of oil put in the just-noted bearing opening so as to be the appropriate amount. Consequently, those who would like to put this technology into practice have to devise on their own a method of making the quantity of oil put in the bearing opening be the appropriate amount.
With the second oil-charging method on the other hand, the fact that the oil has to be injected into the bearings under a vacuum environment means that compared with cases where oil is injected under an atmosphere environment the work is far and away more complicated, wherein finely accurate control of the injection volume is difficult. Incidents of oil leakage and shortening of bearing lifespan that originate in the amount of oil being too much or too little are therefore difficult to keep adequately under control.
Where fluid-dynamic-pressure bearings of the full-filled type are utilized in motors for 3.5-inch or server-class hard disk drives, moreover, with the journal length being longer the volume of oil retained in the fluid-dynamic-pressure bearings will thus increase. The amount of oil to be charged into the bearings in such cases is sometimes greater than the capacity of the taper-seal area provided in the bearing end portion. Likewise, in recent years fluid-dynamic-pressure bearings utilizing sintered, oil-impregnated metals have appeared on the market in motors for hard-disk drives, but the fact that therein a section of the motor sleeve is composed of a porous, sintered metal substance means that the amount of oil to be injected is extremely large, exceeding the capacity of the taper-seal area. In such cases the requisite amount of oil consequently cannot be charged into the bearings using either of the techniques described above-the first method or the second method.
An additional problem is that during volume production, with the individual fluid-dynamic-pressure bearings that are assembled not being identical, dimensional fluctuations will be present in the close-tolerance portions of the bearings, and therefore the optimal amount of oil with which they should be charged will vary subtly. If the oil fill volume is set to what fits design specifications (“preestablished oil volume” hereinafter) and all products are filled with the preestablished oil volume in one-time fillings, products that result in oil-volume excesses or shortfalls will be produced. In other words, with certain products, because the amount of oil that is injected will be less than the optimal oil volume that the bearings intrinsically should retain, after the products are assembled as motors the lifespan of the motors will prove to be shorter than the designed duration. With other products, because the amount of oil will be greater than is optimal, when the products are built into a motor there is a chance that oil leaks will develop when the motor is spun.